Week 6: Prism, Bifocals & Progressives Flashcards
One eye may be looking through the distance portion of a bifocal, while other eye is looking through near zone. How would you resolve this issue?
Seg height can be raised up or down to ensure both eyes reach the seg at the same time when rotating (looking) downwards
What should you do when prescribing vertical prism for bifocals?
If prescribing grind prism, eye will deviate to look towards apex
- Fitting cross should be raised 0.3mm for every dioptre of base down prism
- Lowered 0.3mm for every dioptre of base up prism required
Issue: Prescribing prism at near for bifocals, but not for distance. How would you resolve this issue?
- Prism at near can be achieved by decentering the near segments
- When decentering near segments, ensure reading zone area is large enough to facilitate reading
How to prescribe prism slab-off/bi-prism bifocals
- Incorporate slab-off (i.e. remove base down) into the lens for the eye which needs to be more base up relative to the other eye
What type of lenses can be used for prism slab off?
- Are typically only used in bifocal types which have a straight dividing line between distance & near segments
- E.g. flat-top bifocals (a) and executive bifocals (b)
What is cemented or bonded segments
- These can incorporate any kind of prism or prescription modification in the distance & near portions
- Shape, size & location can be manipulated independently
- Can be used to control differential prism
What is a progressive addition lens?
- A lens having optics that vary in power such that the power gradually increases from the distance, to near portions of the lens
How are Progressive Lens Produced?
- Lens is produced such that the curvature of the front surface is gradually increased as we move further down the lens
- Process is known as optical modulation
What is the ‘add’ in progressive lenses?
- A PAL would permit clear vision at any given distance (up to the set reading distance for that patient)
List the different zones of a PAL
- Distance Portion/Zone
- Progressive Corridor/Zone
- Near Portion/Zone
- Periphery
What is Distance Portion/Zone of a progressive?
- Largest section of the PAL lens, will vary in size depending on the design of the lens
What is Progressive Corridor/Zone of a progressive?
- Area of gradually increasing power allowing vision at intermediate distances
What is Near Portion/Zone of a progressive?
Restricted area of clear near vision
- All of the same power (the distance + add)
What is Periphery of a progressive?
- Side portions which contains unwanted astigmatism
What is unwanted cylinder/distortion in PALs?
- Due to the design of the PAL, this distortion cannot be avoided - it must be managed
- The distortion varies in its amount & orientation depending on the design and prescribed add of the lens
- Amount of distortion increases with an increase in the add prescribed
What are some likely symptoms when one is looking through the periphery of a PAL?
- Distortion
- Blur
- Headaches
- Swimming effect
- Eyestrain
List 3 advantages and disadvantages of a PAL
Advantages:
1. Cosmesis
2. All distances are clear in one pair of specs
3. Can be cheaper than separate pairs
Disadvantages:
1. Lenses are expensive
2. Distortion through lenses
3. Takes time to adapt to lenses
4. Requires careful frame selection
Who Should Wear PALs (list 4)
- Presbyopes
- Esophoric myopes whose script is progressing
- Accommodative strabismus
- Pre-presbyopes with accommodative problems
Who should not wear PALs (list 4)
- Satisfied bifocal wearers
- Limited finances
- Monocular patients
- Low myopes
Where does Lowest Add power occur?
At distance reference point (DRP)
Where is Highest Add power located?
At near reference point (NRP)
List the two different corridor lengths and describe briefly
- Long corridor length = softer design & easier to adapt to
- Short corridor length = enable the wearer to reach an adequate near vision zone, particularly in a shallow eye frame
What are the advantages & disadvantages of short corridors in PALs?
Advantages:
1. More near utility in smaller frames
2. Reduced eye declination during near vision
Disadvantages:
1. Less intermediate vision and mid-range utility
2. More rapidly increasing unwanted astigmatism
3. Distortions at bottom of the lens if the frame is too deep
What does the marking and engravings represent in PALs?
- Locating correct areas for fitting the lens & for power measurements
- Indicating the add of the lens
- Indicating the brand and design of the PAL
List what is on the permanent engravings on PALs
- Lens reference points
- Add
- Near Portion/Zone
- Lens Identification logo
What is the ‘Lens reference points’ in progressive lenses?
Indicate the horizontal meridian of the lens, and as a guide to place further markings on the lens
Where is the ‘add’ located in progressives?
Always located temporally on the lens
What is the ‘Near Portion/Zone’ in progressives?
Restricted area of clear near vision
What is the ‘Lens Identification logo’ in progressives?
Listing the lens identification logos so can identify the brand & design of the lens
What is the ‘Lens Identification logo’ in progressives?
Listing the lens identification logos so can identify the brand & design of the lens
List the lens markings for PALs
- Distance checking zone
- Near checking zone
- Fitting cross
- Major reference point (MRP)
What is the ‘distance checking zone’ in progressives?
The distance prescription of the lens is checked through this zone using a focimeter
What is the ‘near checking zone’ in progressives?
Near prescription can be read through this zone
What is the ‘fitting cross’ in progressives?
The lens should be fitted such that the fitting cross sits in front of the centre of the pupil
What is the ‘major reference point (MRP)’ in progressives & where is it located?
- Any prism contained in the lens is measured at this point
- Located at the beginning of the progressive corridor
What is prism thinning?
- PAL’s can become quite thick due to the increasing curvature in the lower portion of the lens
- Prism thinning is used in order to make the lenses thinner & lighter
- Amount of base down prism applied varies with the add of the PAL
What are the different PAL designs?
- Hard vs Soft designs
- Monodesign vs Multidesign
- Customised designs
- Freeform designs
What is hard design for PAL?
- Contains large distance & near zones, however unwanted cyl is highly concentrated in certain areas, making adaptation to the lens harder
- The power in the progressive zone tends to increase faster, meaning the zone is shorter
What are the advantages & disadvantages of hard design in PAL?
Advantages:
1. Can exclude cyl from certain areas
2. Larger delineated distance and reading zones
3. Shorter progressive channel
Disadvantages:
1. Areas of concentrated distortion
2. Longer and more difficult adaptation
3. Progressive zone shorter
What is soft design for PAL?
- Has smaller distance & near zones, however the transition from near to the periphery is much more gradual
- Smaller amounts of cyl are spread out over a much larger area
Advantages & disadvantages of soft design in PAL?
Advantages:
1. Easier, more rapid adaptation
2. Less distortion of peripherally viewed objects
3. Less ‘swim’ of objects with head movement
4. Dioptric power of unwanted cyl is lower
Disadvantages:
1. Possible slightly compromised distance clarity
2. Longer progressive channel
What is the Monodesign Lens in PALs?
- Chooses a basic design that is used for all add powers offered
What is the Multidesign Lens in PALs?
- For a given type of PAL, a different design is used depending on the add of the lens
Describe aspherisation in PALs
- Allows for flatter lens designs without increasing aberrations (notably oblique astigmatism)
- Improved cosmetics with better quality vision
- Traditional methods for measuring lens power will have shortcomings (i.e. focimetry) will have shortcomings, which require greater trust in the lens manufacturers
Describe Hoyalux iD (Physiological Optics)
- Front surface has vertical progression
- Back surface has horizontal progression
List the different Varilux Physio (Wavefront Analysis)
- Wavefront aberrometry
- High order aberrations
- Digital direct surfacing
Describe Wavefront Aberrometry briefly
- Analyses aberrations generated by the reference spec lens surface
Describe Digital Direct Surfacing (on back surface)
- Takes into account the power created by the front & back lens surfaces
- Back surface is calculated point-by-point to maintain the wavefront created by the front lens surface & creates a “best form Rx” for each wearer
What is extended focus lenses in PAL?
- Have a large field of view for reading, with a restricted field for either the distance, or intermediate vision (opposite to standard PALs)
- A type of reading lens which decreases in power as we move towards the top of the lens
What is Degressive/Occupational Lenses?
- Designed based on near add AND reduction in plus power as you move more superior in the lens
Advantages & disadvantages of Degressive/Occupational Lenses?
- ‘Soft’ design as surface astigmatism redistributed in the distance area
- Wearer obtains a wide central field with very low aberration
- Generally improves posture for the wearer, reducing neck & shoulder strain
Disadvantages:
1. NO DISTANCE – unsuitable for driving
2. Creates ‘artificial far point’ – closer than optical infinity
What are Anti-Fatigue Lenses and who are they suitable for?
- Designed as a distance vision lens with low ‘add’
Suitable for;
- Emerging presbyopes
- Patients with digital eye strain
- Emmetropic patients who require ‘accommodative’
